The disclosure of Japanese Patent Application No. 2000-231989 filed on Jul. 31, 2000, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a vehicular brake control apparatus and a vehicular brake control method for individually controlling brakes that are provided in wheels respectively and stabilizing a running state of the vehicle.
2. Description of Related Art
There has been conventionally known a vehicle behavior control apparatus that individually controls brakes that are provided in wheels respectively to stabilize a running state of the vehicle. For example, Japanese Patent Application Laid-Open No. 8-310366 discloses a technique in which, in the circumstance where the vehicle behavior is about to shift to a spin tendency or a drift-out tendency, braking forces for wheels are adjusted in accordance with an actual state of the vehicle behavior in such a manner as to curb the shift. For example, in a vehicle equipped with a center differential that transmits a driving force from an engine to front-wheel and rear-wheel drive shafts while allowing a difference in rotational speed therebetween, if a differential mechanism of the center differential has been locked through operation by a driver, front-wheel-side and rear-wheel-side rotational speeds are locked with respect to each other. Namely, the sum of wheel speeds of the front-left and front-right wheels and the sum of wheel speeds of the rear-left and rear-right wheels always coincide with each other. Thus, if a braking force has been applied to the turning-wise outside front wheel to reduce a speed thereof to a certain extent, one of the following influences, i.e., (a) an increase in speed of the turning-wise inside front wheel, (b) a decrease in speed of the turning-wise inside rear wheel, and (c) a decrease in speed of the turning-wise inside and outside rear wheels emerges to an extent corresponding to a decrease in rotational speed of the turning-wise outside front wheel. In a front-wheel-drive vehicle or a four-wheel-drive vehicle based on a front-wheel-drive mode, the process of estimating a vehicle speed is performed based on a speed of the turning-wise inside front wheel. Therefore, the estimated vehicle speed is high in the case of (a). This causes a rise in target speed of the turning-wise outside front wheel and thus a decrease in braking force applied to the turning-wise outside front wheel. Further, if the speed of the turning-wise inside rear wheel has decreased and the brake has been applied as in the case of (b), a moment promoting a spin tendency acts on the vehicle body. Moreover, if a braking force has also been applied to the turning-wise outside rear wheel as in the case of (c), the tire side force on the rear-wheel side decreases. This affects controllability of the vehicle behavior.
Thus, if turning behavior control of the vehicle is started in the circumstance where the center differential has been locked, driveability of the vehicle may be affected.
Further, if it has been required to lock the center differential through operation by a driver or the like in the circumstance where such brake control of the vehicle has once been started, it is for instance contemplable to lock the center differential according to the requirement and suspend brake control correspondingly. However, if brake control has been suspended before its completion, the spin/drift-out curbing effect to be exerted by brake control may be degraded.
The invention has been made to address the above-described problems. It is one object of the invention to provide a vehicular brake control apparatus and a vehicular brake control method capable of desirable vehicular brake control taking into account a connecting state between front-wheel and rear-wheel drive shafts, a timing for changing the connecting state, and a timing for performing brake control.
A vehicular brake control apparatus according to a first aspect of the invention comprises (a) a connecting state coupling interposed between a front-wheel drive shaft that transmits a driving force to a front-wheel side and a rear-wheel drive shaft that transmits the driving force to a rear-wheel side, and that changes a connecting state between the front-wheel drive shaft and the rear-wheel drive shaft, (b) brakes, each of which is provided in a corresponding one of wheels to apply a braking force to the corresponding one of the wheels, and (c) a brake controller that performs operation control of the brakes in accordance with a running state of the vehicle and that controls braking forces to be applied to the wheels individually. The brake controller prohibits brake control even if conditions for starting brake control by the brake controller have been fulfilled during a period in which the coupling causes a great connecting strength to act between the front-wheel drive shaft and the rear-wheel drive shaft. The brake controller continues brake control even if the connecting state between the front-wheel drive shaft and the rear-wheel drive shaft has been changed during performance of the brake control such that the great connecting strength acts therebetween.
According to the first aspect, while a great connecting strength acts between the front-wheel and rear-wheel drive shafts (hereinafter referred to as xe2x80x9cbetween both the drive shaftsxe2x80x9d in this section), front-wheel-side and rear-wheel-side rotational speeds are more likely to be locked with respect to each other, in comparison with the case where a small connecting strength acts therebetween. Thus, if brake control has been performed for a specific one of the wheels, rotating states of the other wheels are affected. Therefore, in some cases, the effect to be exerted by the brake controller is not achieved sufficiently. To overcome this problem, brake control is prohibited even if the conditions for starting brake control by the brake controller have been fulfilled during a period while a great connecting strength acts between both the drive shafts.
Further, assuming that the connecting strength between both the drive shafts has become great while brake control is being performed by the brake controller, if brake control is suspended immediately on the ground that the connecting strength has become great, brake control is suspended immediately in the circumstance where the running state of the vehicle is about to be stabilized. Thus, in some cases, the effect of brake control is not achieved sufficiently. To overcome this problem, brake control is continued by the brake controller even if the connecting strength between both the drive shafts has become great while brake control is being performed by the brake controller.
In the above-mentioned first aspect, it may be determined whether or not both the drive shafts have been locked with respect to each other by the coupling, so as to determine whether or not the connecting strength between both the drive shafts has become great.
Further, in the case where the connecting strength between both the drive shafts becomes great temporarily, e.g., for a short period that can be determined in advance, if brake control that is being performed is suspended during that period, the effect of brake control may deteriorate correspondingly. To overcome this problem, brake control may be continued by the brake controller while a great connecting strength acts between both the drive shafts temporarily.
Further, in a vehicle that can change over between two-wheel-drive and four-wheel-drive modes, the connecting state coupling operates during a changeover operation between the two-wheel-drive and four-wheel-drive modes, and a great connecting strength acts between both the drive shafts temporarily. Brake control may also be continued by the brake controller in such a case.
A vehicular brake control apparatus according to a second aspect of the invention comprises (a) a connecting state coupling interposed between a front-wheel drive shaft that transmits a driving force to a front-wheel side and a rear-wheel drive shaft that transmits the driving force to a rear-wheel side, and that changes a connecting state between the front-wheel drive shaft and the rear-wheel drive shaft, (b) brakes, each of which is provided in a corresponding one of wheels to apply a braking force to the corresponding one of the wheels, (c) a controller that performs operation control of the brakes in accordance with a running state of the vehicle and that controls braking forces to be applied to the wheels individually, and restricts changes in the connecting state by the connecting state coupling while brake control is being performed by the controller.
According to the second aspect of the invention, in the circumstance where brake control is being performed by the controller, even if it has been required to perform an operation that necessitates changes in the connecting state between the front-wheel drive shaft and the rear-wheel drive shaft, e.g., to lock the center differential or perform changeover between the two-wheel-drive and the four-wheel-drive modes, changes in the connecting state caused by the connecting state coupling are restricted by the controller. Therefore, changes in the connecting state caused by the controller are restricted, and desirable brake control is performed by the controller.
Further, in a vehicle that can change over between two-wheel-drive and four-wheel-drive modes, the connecting state coupling operates during a changeover operation between the two-wheel-drive and four-wheel-drive modes, and a great connecting strength acts between both the drive shafts temporarily. Thus, while brake control is being performed, the changeover operation between the two-wheel-drive and four-wheel-drive modes may be prohibited by the controller.
Further, both the drive shafts may be prohibited from being locked with respect to each other by the controller even if it has been required, e.g., to lock the center differential while brake control is being performed by the controller.
Further, if it has been required, e.g., to lock the center differential or perform changeover between the two-wheel-drive and four-wheel-drive modes while brake control is being performed by the controller, the required operation may be invalidated (prevented from occurring) through operation of the controller. If the required operation remains invalid even after the running state of the vehicle has been stabilized by brake control, the driver is forced to make the same requirement again. To overcome this problem, the required operation is automatically performed after completion of brake control. Thus, the requirement can be satisfied promptly without forcing the driver to make the same requirement (request) again.
In the above-mentioned first and second aspects of the invention, the controller may individually control braking forces to be applied to the wheels such that turning behavior of the vehicle is stabilized, if turning behavior of the vehicle is in a predetermined state.
Even if the vehicle has assumed a spin or drift-out tendency, this tendency is curbed by the controller constructed as described above. As a result, turning behavior of the vehicle is stabilized.
The aspects of the invention are not to be limited to the vehicular brake control apparatus as described above. According to other aspects of the invention, there is, e.g., provided a vehicle equipped with a brake control apparatus or a vehicular brake control method.